1. Technical Field
The present invention relates to an optical fiber for an optical fiber sensor and a chemical sensor using the same.
2. Description of the Related Art
In recent years, numerous studies into chemical sensors using optical fibers have been conducted due to driving stability, optical connectivity, convenience, and high sensitivity thereof. In particular, optical fiber-based chemical sensor technology using multiple-hole optical fibers having holes in a core area or a cladding area have recently attracted much attention. Various properties such as optical absorption, refractive index, concentration, and density of gas, liquid, or solid filled inside the hole of the optical fiber can be measured by monitoring light guided in the hole of the optical fiber in an optical fiber-based chemical sensor.
Multiple-hole optical fibers used in such a chemical sensor may be generally classified into three groups.
The first group is a photonic band gap fiber in which one hole is formed in a core area and a plurality of hole layers is formed in a cladding area, as shown in FIG. 1. The core area of the photonic band gap fiber through which light passes has a hole shape such that characteristics of gas or liquid filling the core area, such as optical absorption, refractive index, concentration, and density, can be measured with high sensitivity. However, transmission of optical fibers is sensitively influenced by the characteristics of the band gap and the geometrical structure of the hole, and it is very difficult to manufacture the optical fiber and the width of the optical transmission band is relatively very narrow.
The second group is a photonic crystal fiber in which no hole is formed in a core area and a plurality of hole layers is formed only in a cladding area, as shown in FIG. 2. Light is transmitted to a core area due to a difference between effective indexes of refraction between the core area and the cladding area in the photonic crystal fiber. Since the core of the photonic crystal optical fiber is formed of solid materials such as glass and polymer, a spectrum bandwidth of transmitted light is wider than that of the photonic band gap fiber, but the ratio of light passing through the holes of the cladding area to the entire amount of light transmitted through the optical fiber, that is, an evanescent field fraction (EFF), is so low that measurement sensitivity is very low.
The third group is a suspended core fiber 302 by which an evanescent field is increased by reducing a core area by a predetermined size or more. Large holes 304, 306, 308 having a diameter of 5 μm or more are formed as a single layer in a cladding area of the suspended core fiber 302, and a suspending wall 316 supporting the core area is formed between the cladding area and the core area. This structure is called a suspended structure. The structure of the suspended core fiber 302 is distinguished from the structure of a conventional photonic crystal fiber in which small holes approximately having a diameter of 1 μm to 4 μm are formed in a plurality of layers.
Like the photonic crystal fiber, the suspended core fiber has a wide transmission optical bandwidth and a reduced diameter of the core area, so that EFF can be increased by several dozen percent. However, the diameter of the core must be decreased below 1.5 μm in order to obtain 10% or higher EFF and, in this case, optical connectivity of the suspended core fiber to a conventional optical fiber becomes very low. Here, as shown in FIG. 3, the term “core diameter” refers to a diameter of circles adjoining core side surfaces of holes (cladding holes) 304, 306, 308 located in the cladding area.